1. Field of the Invention
This invention relates to fabric-washing compositions. More particularly it relates to an improved low temperature bleaching fabric-washing composition comprising a peroxide compound, a peroxyacid bleach precursor and enzymes.
2. The Related Art
It is well known that active oxygen-releasing peroxide compounds are effective bleaching agents. These compounds are frequently incorporated in detergent compositions for stain and soil removal. They have, however, an important limitation: the activity is extremely temperature-dependent. Thus, active oxygen-releasing bleaches are essentially only practical when the bleaching solution is heated above 60.degree. C. At a bleach solution temperature of about 60.degree. C., extremely high amounts of the active oxygen-releasing compounds must be added to achieve any bleaching effect. This is both economically and practically disadvantageous. As the bleach solution temperature is lowered below 60.degree. C., peroxide compounds, e.g. sodium perborate, are rendered ineffective, regardless of the level of peroxide compound added to the system. The temperature dependency of peroxide compounds is significant because such bleach compounds are commonly used as a detergent adjuvant in textile wash processes that utilize an automatic household washing machine operating at wash water temperatures of below 60.degree. C. Such wash temperatures are utilized because of textile care and energy considerations. Consequently, a constant need has developed for substances which render peroxide compound bleaches more effective at bleach solution temperatures below 60.degree. C. These substances are generally referred to in the art as bleach precursors, promoters or activators.
Typically, the precursor is a reactive compound of the N-acyl or O-acyl type such as a carboxylic acid ester that in alkaline solution containing a source of hydrogen peroxide, e.g. a persalt, such as sodium perborate, will generate the corresponding peroxyacid, which is more reactive than peroxide compounds alone. The reaction involves nucleophilic substitution on to the precursor molecule by perhydroxide anions (HOO.sup.-) and is facilitated by precursors having good leaving groups. Often this reaction is referred to as perhydrolysis. Numerous substances have been proposed in the art as effective bleach precursors, promoters or activators, such as disclosed in a series of articles by Allan H. Gilbert in Detergent Age, June 1967, pages 18-20, July 1967, pages 30-33, and August 1967, pages 26-27 and 67; and further in GB patents 836,988; 907,356; 1,003,310 and 1,519,351; German patent 3,337,921; EP-A-0185522; EP-A-0174132; EP-B-0120591; and U.S. Pat. Nos. 4,412,934 and 4,675,393.
Normally, the precursor is also a hydrolysable material which can react with moisture and alkaline components of the detergent compositions during storage, forming nonreactive products. This reaction, referred to as hydrolysis, causes loss of precursor during storage when incorporated in detergent compositions, the extent of which is highly dependent upon the ease at which the precursor undergoes the hydrolysis reaction.
Various means have been proposed in the art to protect the precursor from the aqueous and alkaline components of the detergent composition during storage. It should be appreciated, however, that the less stable to hydrolysis the precursor is the more difficult it will be to achieve adequate protection.
It is believed that this may be one reason why only a few of the large number of proposed compounds have found commercial exploitation, of which N,N,N',N'-tetraacetylethylene diamine (TAED), belonging to the type of N-acyl precursors, is the one most widely used in practice.
One drawback of TAED, however, is the sluggishness of the peroxyacid release from the reaction with the peroxide compound liberating hydrogen peroxide, such as sodium perborate, sodium percarbonate, sodium persilicate, urea peroxide and the like, resulting in a non-optimal bleaching effect. TAED can thus be classed as a slow-acting precursor, which can be incorporated in enzymatic alkaline detergent compositions without undue stability problems.
Another drawback of TAED is that its solubility in water is rather poor, i.e. somewhere in the region of 1%, which is another reason for the non-optimal bleaching performance of TAED/H.sub.2 O.sub.2 systems.
With the trend towards still lower fabric-washing temperatures, to e.g. 40.degree. C. and below, there is an incentive to improve on the bleaching performance of TAED/peroxide compound systems. One option is to replace TAED by a more reactive precursor, such as for example sodium p-acetoxybenzene sulphonate as disclosed in GB patent 846,798.
A disadvantage of such more reactive precursors, however, is that they tend to (per)hydrolyse more readily than tetraacetylethylene diamine (TAED), and hence suffer from a more severe decomposition problem during storage.
Another disadvantage of more reactive precursors is that they tend to more readily attack enzymes, especially proteolytic enzymes, which as a class is an essential ingredient in the majority of current household fabric-washing compositions.
Consequently, a constant need has developed of possibly new and better substances which render peroxide compound bleaches more effective at bleach solution temperatures in the region of from ambient to about 40.degree. C., without the above drawbacks and disadvantages.